In diverse areas of application, there is a need for determining forces which are applied into components. The forces applied into the components in the axial direction are of particular interest primarily in the case of components in the forms of axles, shafts, or bolts.
An axial force applied into a component can be determined, for example, in that a deformation of the component in the axial direction, which is induced by the introduction of force, is detected by performing a measurement. A plurality of measuring sensors is generally used for this purpose, which sensors are spaced apart from each other in the axial direction along the component and which are utilized for measuring an axial extension and/or an axial compression of the component. There is a problem, in this case, that representative points on the components are not always accessible. Therefore, it is difficult, in part, to position a plurality of measuring sensors at different axial positions.
The object is to eliminate these disadvantages. DE 10 206 679 describes a method, for example, in which the axial force applied into an axle or shaft is determined, in that an annular measuring body, which rests via its two end faces against contact surfaces which are radially offset with respect to each other, is exposed to the axial force. The annular measuring body consequently undergoes bending deformation which is measured by means of strain gauges. The axial force is inferred from the detected deformation of the annular measuring body.
The known method has proven effective for measuring particularly low axial forces. There is also a need, however, to measure greater axial forces, for example, in the range of a few meganewtons or more.
The measurement of relatively great forces can be of interest, for example, in the case of gas turbine systems. In the case of tie-bolts, by means of which the rotor disks are clamped against each other in the axial direction, a correct tie-bolt tension should be set throughout the entire service life, primarily in order to ensure the torque transfer and the mechanical integrity of the rotor even in malfunction situations.
During the first startup of a new system, the tie-bolt tension can be achieved by means of defined hydraulic stretching using a known force and via the elastic elongation of the tie-bolt. It can be desirable to monitor the tie-bolt tension over the course of the service life of the gas turbine.
Specifically, the tie-bolt, onto which the rotor disks of the gas turbine are slid, is fixed in position at its one end and an axial tensile force is applied into the tie-bolt from the free end of the tie-bolt, in that, for example, a classical nut is screwed onto a male thread provided on the free end of the tie-bolt and is tightened against a stationary abutment in the direction of the stationary fixation of the tie-bolt. The design of a gas turbine system comprising tie-bolts can be found, inter alia, in the textbook “Stationäre Gasturbinen” (“Stationary Gas Turbines”) by Ch. Lechner and J. Seume, Springer-Verlag 2003, ISBN 3-540-42831-3.
Since, in the case of classical nuts, the torque required to tighten the nuts increases by a substantial extent as the size of the nuts increases, refined tensioning devices are used instead of the classical nuts in some cases in order to tension components, for which relatively large nuts are required, inter alia, in order to tension tie-bolts in gas turbine systems. These tensioning devices comprise a tension body which is generally cylindrical and has a central axial main threaded bore and a plurality of secondary threaded bores distributed evenly along the circumference of the main threaded bore, and comprises a plurality of tensioning bolts screwed into the secondary threaded bores. In order to load a component, the tension body is screwed onto the free end of the component. In order to apply a load, the tension body is not rotated further, however, but rather the tensioning bolts are tightened against a stationary abutment in the direction of the stationary fixation of the component.
In bolted joints having large diameters, such a tensioning device can absorb the high preloads and distribute them onto the individual tensioning bolts. One embodiment of such a tensioning device is known from DE 699 37 246 T2, for example. The tensioning devices comprising multiple tensioning bolts are also known by the abbreviation MJT (multi-jackbolt tensioner).
It is known, in order to determine the preload introduced into a component by means of tensioning devices comprising multiple tensioning bolts, to determine the preload present at each individual tensioning bolt and to add up the individual values in order to obtain the resultant preload. The preload on each individual tensioning bolt can be determined by means of a differential measurement of the bolt length in a loaded and an unloaded state. For this purpose, an ultrasonic signal is injected—by means of a sensor provided on each bolt head—into the particular bolt, is reflected on the lower end of the bolt, and is captured again by the sensor. The change in length can be determined from the transit-time difference of the ultrasonic signal and the preload can be determined from the change in length.
A disadvantage of the known method for determining the preload is considered to be, in part, that each individual tensioning bolt must be equipped with a separate sensor and, therefore, special equipment must be utilized. This not only requires a relatively great amount of effort, it is also relatively expensive. In addition, the accuracy of this approach, in which a plurality of individual measurements is carried out and the individual values are subsequently added up, is not always satisfactory.
JP S57161526 A describes a measuring device for determining a tension in a bolt, in the case of which a magnetic field sensor is utilized. For this purpose, the magnetic field sensor is mounted on the side of a bolt head of a bolt. A change in the magnetic field can be determined depending on the tension in the bolt. The tension in the bolt can therefore be determined on the basis of the measured values that are determined.
Even though the previous method provides a simple way for determining a tension in a bolt, several disturbance variables—in particular in the case of complex geometries, which is generally the case with a rotor of a gas turbine—can have an incalculable influence on the measured result, however.
U.S. Pat. No. 4,246,780 A describes yet another measuring method for determining the tension in a shaft, wherein a ring comprising strain gauges is mounted on the shaft. In the case of a drive shaft of a propeller, the torsion results in an increase in the circumference of the shaft, which can be determined by way of the strain gauges. The torsional stress in the shaft can therefore be inferred.
The aforementioned measurement arrangement is problematic for the intended application, however, in that the measurement must take place directly within the area under tension. This area is not accessible in the case of a tie-rod of a rotor of a gas turbine, however. Moreover, the aforementioned method only yields a reliable determination of the torsion.